We are limited to 8 starts from SPs per week, and also have a season acquisition limit. Most teams usually only carry 6 or 7 SPs because of this. There are no worthwhile SP available in FA. An additional crappy starter may get lucky and earn a win in a given week, but at the expense of damaging the team's WHIP and ERA. Much more beneficial to play as many closers and dominant middle relievers every day.